.1^1^] Hodgson: Foliar Abscission i)i Citrus 421 



pieces cause them to hang together mechanically for some time after 

 separation has been consiunmated. In testing for abscission time it 

 is necessary to give the shoot a smart jar in order to cause those leaves 

 in which the process has already been completed, to drop. In the 

 moist chamber this is subsequently accomplished automatically by 

 the pressure developed in the further division of the cells of the 

 abscission zone following separation. 



Cytological Behavior Prior to Separation 

 Prior to the initiation of abscission, the tissues on both sides of the 

 zone are plentifully supplied with starch which is either withdrawn 

 or used before the process begins. At this time the cells of the abscis- 

 sion zone are gorged with starch while the tissues on both sides con- 

 tain very little. Chlor-iodide of zinc and potassium iodide-iodine 

 applied to sections cut just prior to the initiation of the process, 

 delimit the zone and show it as a dark purple band. Examination 

 at subsequent stages indicates that the starch content decreases as the 

 process goes on, but even at complete separation the abscission cells 

 contain notable quantities. Following separation, if moisture condi- 

 tions are favorable, the cells constituting the abscission zone resume 

 growth and active division and thus utilize the starch which is stored 

 in them. The reason for the persistence of the starch in the abscission 

 zone when it has disappeared elsewhere is not clear, but it is probable 

 that soon after the application of the stimulus which is to result in 

 abscission, changes occur in the cell walls of the abscission zone which 

 prevent the translocation of the products of starch hydrolysis to 

 adjacent cells. 



The earliest indication of chemical alterations in the cell walls of 

 the abscission zone is evidenced by a decreased power of taking up and 

 holding stains. Thus at six to fifteen hours prior to abscission, when 

 stained with iodine the cell walls of the abscission zone show a lighter 

 color than tliose adjacent (cf. Lloyd, 1916). 



When stained with strong potassium iodide-iodine the walls of the 

 abscission cells take on a delicate greenish hue. On dilution with 

 water this light green fades out and is replaced with a pale blue which 

 is ultimately lost on further dilution. This change from green to blue 

 occurs in all of the cells of the abscission zone and constitutes a very 

 delicate test (cf. Lloyd, 1U16). Just prior to abscission, the cells of 

 the abscission zone when stained over night in dilute methylene blue, 



